1. Field of the Invention
This invention relates to a hydraulic control system for vehicular automatic transmission.
2. Description of the Prior Art
In the vehicular automatic transmission having a fluid coupling type torque converter and a transmission gear mechanism with a plural number of friction elements to establish a number of speed ratios, the gear mechanism is automatically shifted into a speed ratio optimum for the current operating condition by applying and releasing the respective friction elements in predetermined modes of operation. The friction elements are controlled by a hydraulic control system incorporating a number of shift values which are operated according to antagonistically acting forces of a throttle pressure varying in response to the intake throttle opening and a governor pressure varying in response to the vehicle speed. The hydraulic control system of this sort generally incorporates a manual selector valve for manually switching the operating range of the transmission each range defining a speed ratio to which the transmission is shiftable, namely, the maximum speed ratio to which the transmission is shiftable. The operating ranges selectable by the manual selector valve normally include a number of forward ranges such as D-range, 2nd range and L-range. For example, when the manual selector valve is in D-range position, the transmission is shifted over a wide range of speed, from 1st to a maximum speed ratio rated for the transmission, normally 3rd or 4th speed.
In a case where the transmission has four forward speeds including overdrive (4th speed), the conventional hydraulic control system are designed to effect the upshift from 3rd to 4th speed and the downshift from 4th to 3rd speed at shift points which are determined solely by the throttle opening as shown in FIG. 1, holding 3rd speed irrespective of the vehicle speed in an operating condition close to a full throttle opening area A. However, it is desired to effect shifts between 3rd and 4th speeds even in the full throttle opening area in order to prevent overrunning of the engine which occurs when the gear mechanism is shifted from 4th to 3rd speed of a greater reduction ratio due to an abruptly increased throttle opening (the so-called 4-3 kickdown) during overdrive operation, for the purpose of improving the performance quality of the vehicle at high speeds.
In the conventional hydraulic control system, the shift points to and from 3rd and 4th are determined by the acting areas of the throttle and governor pressures on the valve body of the highest 3-4 shift valve, and at the time of kickdown the valve body is operated by the throttle and detent pressures acting against the governor pressure. The governor pressure increases in proportion to the vehicle speed in low and middle speed ranges but its increase is limited in the high speed range due to oil leakage, so that it becomes difficult to obtain a governor pressure which is high enough for effecting the 3-4 upshift in the full throttle area A of FIG. 1 at a point over the ordinary shift points in the lower throttle opening area. On the contrary, if it is arranged to effect the 3-4 upshift in the full throttle opening area A, the shift points in the smaller throttle opening area have to be lowered to unduly low levels.